1. Field of the Invention
The present invention relates to a television tuner.
2. Description of the Related Art
A conventional television tuner will be described by referring to FIGS. 5 and 6. A television tuner comprises: a frequency converter 31 for frequency converting a received television signal to an intermediate frequency signal (hereinbelow, abbreviated as an IF signal) and outputting the IF signal; an intermediate frequency tuning circuit (hereinbelow, abbreviated as an IF tuning circuit) 32 for passing the IF signal outputted from the frequency converter 31; and an intermediate frequency amplifier 33 for amplifying the IF signal and outputting the amplified IF signal.
First, a television signal received by an antenna (not shown) is amplified by an amplifier (not shown) and the amplified signal is supplied to the frequency converter 31. The frequency converter 31 has first and second output terminals 31a and 31b and, although not shown, a voltage controlled oscillator and a mixer. In the mixer, an oscillation signal outputted from the voltage controlled oscillator is mixed with the television signal, thereby performing the frequency conversion and obtaining an IF signal. The IF signals are outputted from the first and second output terminals 31a and 31b in a balanced state. For example, in a television tuner complied with a domestic specification, a sound intermediate frequency SIF is 54.25 MHz, a color intermediate frequency CIF is 55.17 MHz and a picture intermediate frequency PIF is 58.75 MHz.
The IF tuning circuit 32 has a capacitor 32a and a tuning coil 32b. The capacitor 32a and the tuning coil 32b are connected in parallel. One end of the tuning coil 32b is connected to the first output terminal 31a of the frequency converter 31 and the other end of the tuning coil 32b is connected to the second output terminal 31b of the frequency converter 31. A tuning frequency f0 of the IF tuning circuit 32 is set so as to be almost a center frequency (57 MHz in the domestic specification) between the picture intermediate frequency PIF and the color intermediate frequency CIF.
The intermediate frequency amplifier 33 is connected to both ends of the tuning coil 32b via coupling capacitors 34 and 35. The IF signals outputted from the frequency converter 31 pass through the IF tuning circuit 32 and are supplied to the intermediate frequency amplifier 33 in a balanced state. The IF signals are amplified and outputted.
As shown by a solid line in FIG. 6, the level of the IF signal outputted from the IF tuning circuit 32 becomes the maximum value (point A) at the tuning frequency f0. An output characteristic drawing a curve of the level of the IF signal, which is gradually lowered as the frequency becomes higher or lower than the tuning frequency f0 is obtained. The level of the picture intermediate frequency PIF is lower than that at the tuning frequency f0 (point B). Similarly, the level of each of the color intermediate frequency CIF and the sound intermediate frequency SIF is lower than that at the tuning frequency f0 (points C and D).
For example, transistors (not shown) are used for the mixer of the frequency converter 31 and an amplifier (not shown) provided at the front stage of the frequency converter 31. Generally, the gain of the transistor decreases when the ambient temperature rises. In the output characteristic of the IF signal of the frequency converter 31, as shown by a broken line in FIG. 6, the gain decreases as a whole. Consequently, as shown by broken lines in FIG. 6, the levels of the sound intermediate frequency signal SIF and the picture intermediate frequency signal PIF decrease from the points D and B to points Dxe2x80x2 and Bxe2x80x2, respectively. Especially, the decrease in the level of the picture intermediate frequency signal PIF causes a problem such that the S/N ratio of an image displayed on a display of a television deteriorates.
The invention intends to solve the problem and its object is to suppress attenuation of the level of a picture intermediate frequency PIF when the ambient temperature rises.
In order to solve the problem, according to the invention, there is provided a television tuner comprising: a frequency converter for frequency converting a received television signal into an intermediate frequency signal and outputting the intermediate frequency signal; and an intermediate frequency tuning circuit for passing the intermediate frequency signal, wherein a tuning frequency of the intermediate frequency tuning circuit is changed to a higher value in association with a rise in the ambient temperature.
According to the television tuner of the invention, the intermediate frequency tuning circuit has at least a capacitor and a coil, and a capacitor having a negative temperature characteristic is used as the capacitor.
According to the television tuner of the invention, when the tuning frequency of the intermediate frequency tuning circuit is set to an almost center frequency between a picture intermediate frequency and a color intermediate frequency of the television signal at a specific temperature in an ordinary temperature range, the tuning frequency when the ambient temperature rises by 25xc2x0 C. is set to be higher than the frequency which is the almost center between the picture intermediate frequency and the color intermediate frequency of the television signal and lower than the picture intermediate frequency.